A commonly used insulating material for insulated electrical and optical conductors is the halogenated resin, poly(vinyl chloride) (PVC). It is relatively inexpensive, widely available, flexible, and has inherent flame resistant properties. However, there is an increasing desire to reduce or eliminate the use of halogenated resins in insulating layers due to their negative impact on the environment. Many jurisdictions now mandate or are planning to mandate a decrease in the use of halogenated materials such as PVC. Therefore there is a continuing need to develop new halogen-free insulation materials.
One stringent standard for performance of insulated electrical cables is set forth in Harmonization Document HD21.14 S1, “Cables of rated voltage up to and including 450/750 V and having thermoplastic insulation. Part 14: Flexible cables (cords), insulated and sheathed with halogen-free thermoplastic compounds”, May 2003. A composition comprising polyethylene and at least 60 weight percent magnesium dihydroxide meets the technical requirements of HD21.14 S1, but users have complained that insulation made from this composition is too rigid and exhibits a scratch-sensitive surface appearance, while the composition itself exhibits a relatively high density (about 1.5 gram per centimeter3) and a high melt viscosity that makes it difficult to use for insulation fabrication.
Recent research has demonstrated that certain halogen-free poly(arylene ether) compositions can possess the physical and flame retardant properties needed for use as insulation in some wire and cable applications. See, for example, U.S. Patent Application Publication Nos. US 2006/0106139 A1, US 2006/0182967 A1, and US 2007/0261878 A1 of Kosaka et al., and US 2006/0131059 A1 of Xu et al. However, existing poly(arylene ether) compositions that meet the physical requirements of HD21.14 S have been challenged to meet the low-smoke requirements of that standard.
There remains a need for thermoplastic compositions satisfy the requirements of the HD21.14 S standard while exhibiting one or more improvements such as greater flexibility, reduced scratch sensitivity, reduced density, and reduced melt viscosity.